Heat responsive marking sheets



HEAT RESPONSIVE MARKING SHEETS Filed Nov. 19, 1962 CO-CRYSTAL ADDUCT OF AN AMINE AND A HYDROXYPHENOL COMPOUND SULFUR-CONTAINING REDUCING AGENT SALT OF cu, PB,AG 0R HG SUPPK JRT INVENTORS MICHAEL F. BAUMANN WILLIAM R.LAWTON A TORNEYS United States Patent 3,185,585 HEAT RESPONSIVE MARKING SHEETS Michael F. Baumann, Hudson, and William R. Lawton, Nashua, NH, assignors to Nashua Corporation, Nashua, N.H., a corporation of Delaware Filed Nov. 19, 1962, Ser. No. 238,710 4 Claims. (Cl. 11736.9)

This invention relates to a heat-marking process and is concerned with a copy sheet characterized by its ability to form a strong black image of a master copy whenever the sheet is exposed through the master copy to heat, or whenever its surface is heated by a marking stylus such as are commonly used in recording machines.

For brevity and convenience,1we use the term thermographic process to describe that process in which the copy sheet is heated not only directly as by a heated stylus but also is heated selectively usually by infra-red radiation. The absorption of radiant energy by the ink on the master copy converts that energy into sensible heat and thereby effects a color change in those portions of the copy sheet which lie under the original indica. In this manner a replica of the printed or written page is produced.

One of the difficulties of the thermographic process is that the color change which is produced by heating often produces an image of brownish or bluish shade. Frequently, the image is not intense so that the sheets do not have the clear contrasting readability of a photographic or photostatic copy. We have discovered a class of compounds and the appropriate reactions applicable to those compounds which permit a copy sheet, when exposed to heat under the conditions of the thermographic process, to

produce an intense black image on a contrasting white background. i

The present invention is a continuation of work begun in the application of William R. Lawton and Eugene F. Lopez, Serial No. 808,012, filed April 22, 1959, now U.S. Patent No. 3,076,707, and theapplication of Lawton, Serial No. 206,186, filed June 29, 1962. In the above applications, it was disclosed that the development of color upon the application of heat could be secured by the heat decomposition of an inclusion compound or cocrystal of an amine and either a bisphenol or a hydroxyphenol substituted chroman. The amine which was released by heat from these co-crystals, which are inert and nonreactive at atmospheric temperatures, brought about or developed a colored image from its reaction with a diazo composition which had been coated on the sheet.

We have discovered that if a salt of copper, lead, silver, or mercury is mixed with a strong sulfur-containing reducing agent and if this mixture in intimate contact with one of the above heat-decomposable amine-containing inclusion compounds is spread upon a supporting sheet, an intense black image will be formed wherever the sheet has been heated.

The course of this reactionis not completely understood, and the nature of the intense black end-product is not fully known. Whether or not this end-product is the reduced form of the metal and oxide of the metal or possibly the sulfide isas yet undetermined. It is significant, however, that the intense black color has only been found to develop when combined sulfur is present in the molecule of the reducing agent.

In one embodiment of the invention, thermographic copying sheets are produced by combining all ingredients in a fine grain, 10% solids dispersion in the trichlorethylene in which 2 /2 of polyvinyl butyral (Butva-r B-76) was added as a binder. The coating is spread by conventional coating machinery at pounds (dry weight) per ream (24" x 36" x 500 sheets). The weight of the coat- 3,185,585 Patented May 25, 1965 Ice taining a water soluble adhesive such as polyvinyl alcohol, carboxymethyl cellulose, hydroxyethyl cellulose, gums, etc., and spread as before. When this coating is dried, the second coating of the metal salt and the reducing agent in a solvent and adhesive is spread upon the first dried coat.

Solid heat-dissociable co-crystal adducts containing bisphenols or hydroxyphenol substituted chromans and amines are well known. Many bisphenol adducts are described in US. Patent No. 2,829,175. The hydroxyphenol substituted chroman adducts are described by Cramer, Einschlussverbindungen (Springer-Verlag, Berlin, 1954). Inasmuch as directions for producing the bisphenols and the hydroxyphenol substituted chroman adducts appear in the above identified applications of Lawton and Lopez, and William R. Lawton, and both applications are assigned to the assignee of this invention, the entire disclosures of US. Patent application, Serial No. 808,012, filed April 22, 1959, now US. Patent No. 3,076,707, and Serial No. 206,186, filed June 29, 1962, are herein incorporated by reference.

We prefer to use adducts which are light colored for thereby the background color is lighter and provides a maximum contrast for the colored marks. Additionally we prefer to use adducts which dissociate in the temperature range of about 50-150 C. when in a coated composition with the metal salt. By so doing, we provide a heat-marking sheet which is responsive to the practicable operating range of temperatures obtainable in the thermographic process.

Of the two classes of adducts bisphenols and hydroxyphenol substituted chromansthe chromans as a group possess the lower dissociation temperatures. Preferentially, therefore, we employ this class of materials whenever lower temperatures are necessary or where higher speeds in the copying process are needed. Additionally, we find it preferable to make use of amines which normally are liquid, since the released liquid amine provides a maximum degree of intimate admixing of the reactants after the heat dissociation of the adducts. Thus a complete color-forming reaction is secured.

The accompanying drawing illustrates schematically a heat sensitive sheet corresponding to this invention. The

- sheet is represented as being in cross section.

PREPARATION OF THE BISPHENOL ADDUCTS Example 1 A co-crystal adduct is formed by combining:

Parts Tetrachlorobisphenol A (TCBA) 183 Ethylene diamine (EDA) 30 Toluene 364 by dissolving the tetrachlorobisphenol A in the toluene at about 150 F. adding the ethylene diamine and stirring for about 30 minutes. Thereafter the mixture is chilled to about -30 F. and held at that temperature for about 14 hours (overnight). During this time a precipitate forms. The mass is filtered and the precipitate is washed thoroughly with N-hexane at about room temperature (70 F.) and dried. The solid crystal product is the co-crystal adduct of tetrachlorobisphenol A and ethylene diamine and has a M.P. 208-210 C. A dispersion of this co-crystal adduct is then made by ball milling 10 parts of the adduct with parts of a 2.5%

3 solution of polyvinyl butyral (Butvar-B-76) in trichloroethylene.

In the foregoing example, the bisphenol used was tetrachlorobisphenol A (4,4-isopropylidene)-bis-(2,6-dichlorophenol). We have found that this material is effective in the formation of numerous of the co-crystal adducts which dissociate at temperatures within the range of about 50-150 C., but other bisphenols are available and are useful. These other bisphenol compounds may be employed in the preparation of solid heat-dissociable co-crystal adducts with amines. Compounds which are preferred include tetrabromobisphenol A; 4,4'isopropylidene-bisphenol; 2,2-methylene-bis (4-methyl-6-butylphenol); 4,4-thio-bis-(6-tert. butyl-m-cresol); 4,4'-butylidene-bis-(6-tert. butyl-m-cresol); 2,2'-thio-bis-(4,6-dichlorophenol), 2,2 methyl 4,4 bis (3,4,6 trichlorophenol), and other bisphenol compounds which may be used in this manner are set forth in the previously mentioned Patent 2,829,175.

In addition to ethylene diamine other amines are useful, among which may be mentioned triethanolamine, ethanolamine, cyclohexyl amine, tributylamine, di-n-propylamine, isopropylamine.

PREPARATION OF THE CHROMAN ADDUCTS Example 2 A hydroxyphenol substituted chroman, namely 2-hydroxy-2,4,4,7,4'-pentamethylfiavan is prepared according to a procedure described by Baker and Besley in J. Chem. Soc., 1951, 1102-6. A mixture of m-cresol (440 g.) and acetone is saturated with gaseous HCl without cooling and kept at 40 C. for two days with the exclusion of water. The dark product is added to an excess of 20% sodium hydroxide solution, a layer of ether is poured on the surface, and the mixture is vigorously stirred causing the ether addition product of the dimeride to crystallize. After standing overnight in an open vessel the solid is collected, washed thoroughly with water, dissolved in hexane, and dried with calcium chloride. The solution is filtered and a small amount of ether is added to the solution. The other complex of the flavan compound separates in large, colorless, thick rhombic prisms having a melting point of 76-77 C. A flavan co-crystal adduct is prepared by mixing fifteen parts of a hexane solution of the fiavan ether complex with 7.5 parts of dicyclohexylamine, chilling the mixture overnight, and filtering the solid crystalline adduct product, Ml. 103-105" C.

While this material is one which we have found to be effective in the formation of numerous co-crystal adducts which exhibit dissociation temperatures within the range of about 50150 C., it is not the only available or useful hydroxyphenol substituted chroman. Numerous other hydroxyphenol substituted chromans may be used and among these which we prefer are 7,4- diethyl 2' hydroxy 2,4,4 trimethyl fiavan; cyclohexanespiro 4 (2' hydroxy 7,4 dimethyl 2,3 tetramethylene) flavan; 2'-hydroxy-2,4,4,6,7,4,5-heptamethylflavan; 2-hydroxy-2,4,6,5'-pentamethyl flavan; 4-p-hydroxyphenyl-Z,2,4-trimethyl chroman. As in the case of bisphenols, many amines are suitable, the most effective being the alcohol amines.

PREPARATION OF THE METAL SALT-REDUCING AGENT MIXTURE Example 3 Separate ball milled dispersions are prepared of (a) 10 parts of copper oxalate in 90 parts of a 2.5% solution of polyvinyl butyral in trichloroethylene, (b) 10 parts zinc formaldehyde sulfoxylate in 90 parts of a 2.5 polyvinyl butyral solution, (c) 10 parts of TCBA/DEA adduct (as prepared according to Example #1) in 90 parts of the polyvinyl butyral solution. These separate dispersions are ball milled overnight until a de sired particle size is obtained. The dispersions are then combined in the following ratios: copper oxalate 2 parts, zinc formaldehyde sulfoxylate 1 part, TCBA/ DEA 2 parts. The composition is mixed thoroughly and coated to a dry coating weight of about 4 to 6 pounds per ream on a glassine or similar thin base paper. The dry coated sheet will provide a medium for dry thermographic copying processes.

Example 4 The procedure of Example 3 is followed except that 10 parts of lead oxalate (PbC O is substituted for the copper oxalate.

Other lead salts which may be substituted for the oxalate are: lead stearate, lead chloride, lead formate, and lead benzoate.

Example 5 The procedure of Example 3 is followed except that 10 parts of mercuric thiocyanate is substituted for the copper oxalate.

The limitation on the usefulness of silver salts is light sensitivity. If the intended use of the sheet makes light sensitivity unimportant, a number of silver salts can be used and will produce a dense, black image.

Example 6 The procedure of Example 3 is followed except that 10 parts of silver oxalate is substituted for the copper oxalate. We prefer, however, to use compounds of silver which are relatively light-insensitive.

Example 7 A complex of silver nitrate and thiourea is prepared in the following manner. 0.10 mole (16.9 grams) of silver nitrate is dissolved in 25 grams of water. 0.3 mole of thiourea (22.9 grams) is dissolved in 200 grams of water with slight heating. The solution of silver nitrate is slowly added to the solution of thiourea with agitation, and a white flulfy precipitate is obtained, filtered, and washed with water and throughly dried. The precipitate is the complex silver nitrate-thiourea.

Example 8 A coating composition is prepared by dispersing 20 parts of the co-crystal adduct of Example 2 in 100 parts of a 2% solution of polyvinyl alcohol in water. The mixture is ball-milled to obtain a desired particle size and coated on a sheet of glassine (25 lbs. per ream 24" x 36 x 500 sheets) to a dry weight of 4 lbs. per ream.

Example 9 Example 10 Separate ball-mill dispersion are prepared of (a) 10 parts of copper oxalate in parts of a 2.5% solution of polyvinyl butyral in trichloroethylene and (b) 10 parts of Zinc formaldehyde sulfoxalate in 90 parts of a 2.5 polyvinyl butyral solution in trichloroethylene. These separate dispersions are ball-milled overnight or until a desired particle size is obtained. The dispersions are then combined in the following proportions: copper oxalate 2 parts, zinc formaldehyde sulfoxalate, 1 part.

This composition is overcoated on the dried sheet of Example 8 to a dry weight of 3 (24" x 36" x 500 sheets).

REDUCING AGENT to 4 lbs. per ream The reducing agent may be one of a number of strong sublime when subjected to an increase in temperature. We

prefer zinc formaldehyde sulfoxalate (Decroline X-4, Allied Chemical Co.) since it is effective and stable.

As a general procedure the respective amounts of reacting materialsshould be such as to permit mutual reaction with a total amount suflicient to produce a clear, contrasting mank wherever the sheet is heated. The quantities and proportions may vary considerably but may be readily determined by a routine test.

The supporting sheet may be any of the numerous thin copy base materials, i.e. of rag stock, glassine, etc. When the thermographic apparatus is arranged to produce front printing, i.e. reflex printing where the copy sheet is interposed between the radiant heat source and the graphic original, the backing must be transparent to radiant heat. For direct reproduction or whenever the material is to be used for recording purposes, any sufficiently flexible material is suitable.

We claim:

1. A heat-responsive copy sheet adapted to change visibly to a contrasting color wherever it is subjected to localized heating to a temperature of between about 50 and 150 C. comprising in combination (1) a supporting sheet carrying a coating comprising (2) a solid, normally stable, co-crystal adduct comprising the reaction product of an amine and an hydroxyphenol compounds elected from the class consisting of bisphenols and hydroxyphenol substituted chromans which is heat dissociable in the temperature range of C. C.,

(3) a strong, sulphur-containing reducing agent, and

(4) a salt of a metal selected from the group consisting of copper, lead, silver, and mercury.

2. A heat-responsive copy sheet as claimed in claim 1 wherein the sulfuncontaining reducing agent is zinc formaldehyde sulfoxalate.

3. A heat-responsive sheet as claimed in claim 1 wherein the coating on the sheet comprises copper oxalate, zinc formaldehyde sulfoxalate, the co-crystalline complex of a bisphenol and an alcohol amine, and an adhesive binder.

4. A heat-responsive sheets as claimed in claim 1 wherein the coating comprises a first layer consisting of a finely divided complex of an amine and a hydroxy phenol substituted chroman, and a water soluble adhesive binder, and a second layer overlying the said first layer comprising a finely divided dispersion of a metal salt selected from the class consisting of the metal salts of copper, lead, silver, and mercury, and an adhesive binder.

References Cited by the Examiner UNITED STATES PATENTS 2,813,042 11/57 Gordon et al. 117-368 2,995,466 8/61 Sorensen 117368 3,024,362 3/62 Sus et al. 117368 3,031,329 4/62! Wingert ll7--368 3,076,707 2/63 Lawton et al. 117-368 RICHARD D. NEVI-US, Primary Examiner.

MURRAY KA-TZ, Examiner. 

1. A HEAT-RESPONSIVE COPY SHEET ADAPTED TO CHANGE VISIBLY TO A CONTRASTING COLOR WHEREVER IT IS SUBJECTED TO LOCALIZED HEATING TO A TEMPERATURE OF BETWEEN ABOUT 50* TO 150*C. COMPRISING IN COMBINATION (1) A SUPPORTING SHEET CARRYING A COATING COMPRISING (2) A SOLID, NORMALLY STABLE, CO-CRYSTAL ADDUCT COMPRISING THE REACTION PRODUCT OF AN AMINE AND AN HDYROXYPHENOL COMPOUNDS ELECTED FROM THE CLASS CONSISTING OF BISPHENOLS AND HYDROXYPHENOL SUBSTITUTED CHROMANS WHICH IS HEAT DISSOCIABLE IN THE TEMPERATURE RANGE OF 50*C.-150*C., (3) A STRONG, SULPHUR-CONTAINING REDUCING AGENT, AND (4) A SALT OF A METAL SELECTED FROM THE GROUP CONSISTING OF COPPER, LEAD, SILVER AND MERCURY. 